A very singular muscle: the secondary muscle of chaetognaths

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Abstract
Chaetognaths show in their trunk and tail a type of muscle fibre that is at present unique in the animal kingdom. It was identified and first described by Grassi in 1883, who termed it the secondary muscle. It is less than 1% of the tissue wall volume. This muscle forms four longitudinal bands, two mediodorsal and ventral and two lateral, suggesting an antagonistic functional role. It is related to myoepithelial cells, which may adjust the tonus of the hydroskeleton. The secondary muscle shows a unique mode of cross-striation: two sarcomere types (s1 and s2) alternate in series along the longitudinal axis of the contractile apparatus. Successive Z bands are common to these two sarcomeres types, s1 sarcomeres have irregular A bands, about 1.7 μm in length; their loose architecture, poor lateral cohesion and F1:F2 = 1:9 or 1:10, probably reflects a slow mode of functioning. s2 sarcomeres are pleomorphic. They are composed of filaments assembled laterally in various ways and embedded in a loose matrix; they have no A bands, their length can vary from < 1 μm to > 10 μm without apparent change in myofibrillar density. The functioning of s1 sarcomeres is in accordance with the sliding filament model, but that of the s2 functioning is not; the s2 sarcomeres look like stress-fibres. These two sarcomere types seem to have a relatively independent function. It is suggested that the chaetognath secondary muscle shows a contractile architecture partly related to more general cytoskeletal structures and partly related to a true myofibrillar contractile apparatus. It is a highly dynamic contractile architecture which seems to be able to modulate strongly the muscular contraction. It profoundly differs from all other chaetognath muscles. This muscle is peculiar to chaetognaths; it does not assist in relating them to phyla.